Various pyrazolone derivatives have been known as magenta color image forming couplers (referred to hereinafter simply as "magenta couplers"). However, these pyrazolone derivatives generally have low color forming efficiency (ratio of conversion of the coupler into a dye) when contained in photographic light-sensitive materials, and so-called 4-equivalent couplers, in which the coupling active position is not substituted, usually form only about 1/2 mol of dye per mol of the coupler.
To improve color forming efficiency, so-called 2-equivalent magenta couplers have been known, in which a substituent is introduced into the coupling active position of a pyrazolone type magenta coupler, and the substituent splits off in the color development step. Examples of such couplers are disclosed, for instance, in U.S. Pat. Nos. 3,311,476, 3,419,391, 3,617,291, 3,926,631, etc. Further, magenta couplers in which a substituent is connected to the coupling active position through a sulfur atom (hereinafter referred to as "thio-releasing type") are described in U.S. Pat. No. 3,214,437 (a thiocyano group), U.S. Pat. No. 4,032,346 (an acylthio group or a thioacylthio group), U.S. Pat. Nos. 3,227,554 and 3,701,783 and British Pat. No. 1,494,777 (an arylthio group or a heterocyclic thio group), and West German Patent Application (OLS) No. 2,944,601 (an alkylthio group).
It has been found that when couplers having an arylthio group at the coupling active position among the magenta couplers described in U.S. Pat. Nos. 3,227,554 and 3,701,783 are used in a color photographic light-sensitive material and color images are formed, the light fastness of the color images does not completely satisfy the color photographic light-sensitive materials.
Also, it has been found that when magenta couplers which release an arylthio group as described in British Pat. No. 1,494,777 are used in a color photographic light-sensitive material and color images are formed, the light fastness of the color images is insufficient.
Magenta couplers which release an arylthio group as described in U.S. Pat. No. 4,351,897 are distinguishable in view of overcoming these known defects described above.
However, the photographic light-sensitive materials containing these known magenta couplers in which the 4-position of a pyrazolone nucleus is substituted with a sulfur atom have a tendency to cause desensitization and/or a decrease in density of magenta color images upon color development processing. In particular, such a tendency becomes remarkably strong in the cases where a large amount of the photographic light-sensitive materials are continuously processed, for example, where a large amount of the photographic light-sensitive materials are commercially processed in color processing laboratories. This problem is a severe obstacle in practical use.